An air bag comprises an inflatable bag and means for inflating the bag. Air bags are highly desired life saving devices that have performed well in many accidents and saved many lives. However, the bag must be inflated in a very brief time such as 1/30 of a second that requires rapid movement of the bag from a stored and compacted state to a fully inflated state. The rapid deployment of the bag involves great force. A deploying bag can injure a person during the early phases of deployment if the person is very close to where the airbag is stored. Another hazardous situation is when the occupant is a baby in a rear-facing baby seat. It is also desired to inhibit deployment if there is no person in the seat. Much effort has gone into developing systems for characterizing the occupant and ascertaining the occupant position to meet this need. Proposed systems attempt to ascertain the distance from the inflator to the seat occupant and systems using sonic and optical ranging for that purpose are well known. These systems are deficient in that they cannot reliably distinguish between an occupant and other things such as road maps, beverage cups, packages and voluminous clothing that cause indications that the occupant is near the inflator. Certain prior art systems operate to measure the distance from the inflator to the occupant, presumably because that is the physical variable most easily related to the potential for injury.
Many vehicles include an accelerometer located in the passenger compartment for sensing the deceleration of a crash. These accelerometers are incorporated in sensing and diagnostic modules or “SDMs”, which are decision making centers for the vehicle occupant protection system. The output of the accelerometer may be integrated by an analog circuit or a microprocessor in the SDM to compute a difference between the velocity the vehicle was traveling before a crash and the velocity of the passenger compartment during the crash. The integral of the accelerometer output may be integrated again to obtain the second integral of the deceleration, which is the displacement of a free body from its initial position relative to the vehicle. An occupant not wearing a seat belt is, to a good approximation, a free body. Therefore, this calculation provides the distance an unbelted occupant has moved from his or her initial position at any time during the crash. Vehicles typically include seat belt latched sensors for indicating if seat belts are latched.
Ultrasonic distance measurement based on measuring a time period beginning when sound is generated by a sound emitter and ending when an echo from a object at the distance to be measured is received by a receiver located at a point near the sound emitter is well known and has been used for many years in such as focusing systems for cameras. Using ultrasonic distance measurement to measure the distance from the back of a vehicle seat to the back of a seat occupant works well at larger distances that provide time for vibrations excited during the sound transmission to subside and leave the receiver responsive to low intensity sound.
Position and angle sensors are in commercial production for sensing the position of a seat on its track and the angle the seat back is reclined.
Capacitive proximity sensors have been well known for many years and have many successful applications. In addition to measuring capacitance, the Q of the capacitance may be measured to provide additional information about the nature of the material being detected. Some materials including materials containing water tend to significantly reduce the Q of the sensed capacitance.
Capacitance between two electrical conductors is a property that is measured by the amount of separated charge that can be stored on the conductors per unit change of voltage between the conductors. Capacitance in electric circuits is deliberately provided by a device called a capacitor. A capacitor is two electrodes, often called “plates” for historical reasons, of conducting material electrically insulated from each other and separated by a dielectric. Any two members made of conducting material (herein called electrodes) and insulated from each other operate as a capacitor and technology for measuring the properties of a capacitor may be applied thereto.
Except for the leakage (usually small) through the dielectric, no current flows through a capacitor (i.e. from electrode to electrode) when it is subject to a constant voltage. Alternating current will pass readily, however, and is called displacement current.
When alternating voltage is applied between the electrodes of a capacitor there is energy loss in the capacitor. One measure of the loss is the Q or “quality factor” of a capacitor, which is 2π times the ratio of the maximum energy stored to the energy dissipated during one cycle of the voltage applied to the capacitor. Other measures of the loss are the “power factor” and “dissipation factor” which are interrelated by the expressions presented below. Both the capacitance and the Q of a capacitor are strongly influenced by the dielectric.
Ignoring the self inductance of the lead wires, a capacitor is conventionally viewed as a pure capacitance C having capacitive reactance XC which is a measure of opposition to flow of an alternating current of frequency f given by:       x    C    =      1          2      ·      π      ·      f      ·      C      in series with an energy dissipating resistance RC and the combination has an impedanceZ=√{square root over (RC2+XC2)}
In certain capacitors there is a resistance in parallel with the capacitor. A well known example is electrolytic capacitors that have significant series resistance and may have significant leakage through the electrolyte. In automobile seat occupant sensors steps are taken to insulate the capacitor electrodes to prevent leakage across the capacitor. Therefore, a model comprising a zero loss capacitance C in series with an energy dissipating resistance RC is believed to be most like actual seat occupant sensing systems.
The Power Factor (PF) is defined as the ratio of the effective series resistance RC to the impedance Z and is usually expressed as a percentage.
The Dissipation Factor (DF) is the ratio of the effective series resistance RC to capacitive reactance XC and is usually expressed as a percentage. The DF and PF are essentially equal when the PF is 10 percent or less.
The Quality Factor (Q) is a figure of merit and is the reciprocal of the dissipation factor DF, Q=XC/RC.
Circuits for measuring capacitance and the Q of a capacitor are well known and are incorporated in many commercially available measuring instruments and are implemented on single silicon semiconductor dice by several makers.
The concept of the impedance of a capacitor leads to measuring the capacitance of a capacitor by applying an alternating current voltage to the capacitor and measuring the displacement current through the capacitor. The impedance Z is equal to the applied voltage V divided by the displacement current ID: Z=V/ID. The current leads the voltage by a phase angle (phi).
If the Q of the capacitor is large, RC can be ignored, Z and XC are approximately equal, and the capacitance is obtained directly from the displacement current ID and the frequency f and voltage V of the applied alternating current   C  =            I      D              2      ·      π      ·      f      ·      V      
For smaller Q or greater precision, the capacitive reactance XC, the equivalent resistance RC, and the capacitance C are calculated from:
XC=Z·sin(phi) RC=Z·cos(phi)  C  =      1          2      ·      π      ·      f      ·              X        C            
Page 322 of the book Electrical Instruments and Measurements by Walter Kidwell and published in 1969 by McGraw-Hill, Inc. states that “Capacitance can be measured in a number of ways”. It further states “Generally, there are two practical ways of measuring capacitance:”
“1. Absolute measurements in terms of other electrical units.”
“2. Comparison methods, where the unknown capacitor is compared with a known standard that has been previously calibrated.”
“Bridge methods are in the latter category, and it is to these methods that we shall confine our discussion on the following pages.”
The aforementioned book by Walter Kidwell then proceeds to illustrate a Wien Bridge”, a “Generalized capacitance bridge”, a “Five terminal bridge network”, a simplified method of connecting a three terminal network, a “Schering bridge”, a “shielded Schering bridge”, and a bridge having a “Wagner ground”.
All of the capacitance bridges share the common feature of presenting an alternating current signal to the series combination of an unknown capacitor and a first known element(s) of the bridge. Other elements of the bridge with known properties form a second voltage divider producing a signal for comparison with the signal at the junction between the unknown capacitor and the first known element of the bridge. When the bridge is balanced, the amplitudes and phases of currents in all of the elements of the bridge can be calculated relative to the amplitude and phase of the alternating current signal. Therefore, the illustrated capacitance bridges operate by a process that determines the amplitude and phase shift of the current in the capacitor being tested.
The following illustrates, by using the examples of the Wien bridge and the Schering bridge, cases of capacitance measurement accomplished by applying a signal to one electrode of a capacitor and observing the signal at the other electrode of the capacitor.
FIG. 3 illustrates a Wien bridge. It is taken from FIGS. 10–15 of the aforementioned book Electrical Instruments and Measurements. Pages 322 through 329 of this book discuss measuring capacitance. In FIG. 3 the parallel combination of Cd and Rd represent respectively the energy conserving and energy dissipating properties of the capacitance to be measured. In FIG. 3 signal generator 212 provides a signal to electrode 214 of the unknown capacitor Cd. The signal at electrode 216 of the unknown capacitor represented by the parallel combination of Cd and Rd is observed and compared with a comparison signal provided by the resistors Ra and Rb by such as the illustrated headphones. The components Rc and Cc are varied to achieve a balance wherein there is no signal across the headphones 218. Peculiar to the Wien bridge, the energy conserving property Cd of the unknown capacitance can be determined from the frequency of the signal from the signal generator and the values of the resistors in the bridge without knowing the capacitance of the adjustable capacitor.
When the Wien bridge of FIG. 3 is balanced as indicated by no signal at headphones 218 the displacement current through the unknown capacitor represented by the capacitance Cd in parallel with resistance Rd is equal to the displacement current through the variable capacitor Cc. Both the magnitude and the phase of the displacement current ID through the variable capacitor Cc can be determined by the following process. The voltage across the series combination of Cc and Rc is equal to the voltage E from signal generator 212 times the ratio Rb/(Ra+Rb). The current in the right side of the bridge is equal to the voltage across the series combination of Cc and Rc divided by the impedance of the series combination of Cc and Rc where XCc is computed from XCc=1/(2·πf·Cc). Accordingly, the displacement current and its phase are given by:       I    D    =                              E          ·                      R            b                                                (                                          R                a                            +                              R                b                                      )                    ·                                                    R                C                2                            +                              X                Cc                2                                                        ⁢                          ⁢      phi        =                  tan                  -          1                    ⁢                                                  R              c              2                        +                          X              Cc              2                                                R          c                    
The displacement current through the capacitor being tested (represented by the parallel combination of Cd and Rd) is ID and the phase shift is phi because the two right legs of the bridge carry the same current. The voltage Ed across the capacitor being tested is the voltage E from signal generator 212 times the ratio Ra/(Ra+Rb):       E    d    =            E      ·              R        a                            R        a            +              R        b            
With the voltage Ed, current ID, and phase shift phi across the capacitor being tested known, the impedance Zc can be calculated from Zc=resistance Rc times the ratio Ra/Rb. as described hereinabove the capacitive reactance XCc, the equivalent resistance RCc, and the capacitance Cc are calculated from:
XCc=Zc·sin(phi) RCc=Z·cos(phi)      C    c    =      1          2      ·      π      ·      f      ·              X        Cc            
The Quality Factor (Q) is calculated from Qc=XCc/RCc.
FIG. 4 illustrates a Schering bridge. It is taken from FIGS. 10–19 of the aforementioned book Electrical Instruments and Measurements. C2, illustrated in FIG. 4, is an unknown capacitor that may have qualities that cause energy loss. In FIG. 4 signal generator 312 provides a signal to electrode 314 of capacitor C2. The signal at electrode 316 of capacitor C2 is observed and compared with a comparison signal provided by capacitor C1 in series with the parallel combination of capacitor C4 and resistor R4 by such as the illustrated galvanometer 318. The components C4 and R4 are varied to achieve a balance wherein there is no signal across the galvanometer 318.
When the Schering bridge of FIG. 3 is balanced as indicated by no signal at or current through galvanometer 318 the displacement current through the unknown capacitor represented by the capacitance C2 is equal to the current through the resistor R3. The phase shift of the current in the circuit including unknown capacitor C2 is the same as the phase shift in the left leg of the bridge. Both the magnitude and the phase of the displacement current I4 through the fixed capacitor C1, and therefore the voltage across capacitor C1 can be determined by application of elementary circuit theory to the parallel combination of variable resistor R4 and variable capacitor C4 in series with capacitor C1. The voltage across unknown capacitor C2 is equal to the voltage across capacitor C1. The displacement current I2 through unknown capacitor C2 is calculated from I2=ER3/R3 where ER3 is the voltage across resistor R3, which is known because it is the same as the voltage across the parallel combination of variable resistor R4 and variable capacitor C4, which was determined from elementary circuit theory.
With the voltage E2, current I2, and phase shift phi across the capacitor C2 known, the impedance Zright of the right leg of the bridge is calculated from Zright=the voltage E from signal generator 312 divided by the current I2. As described hereinabove the capacitive reactance XC2, the equivalent resistance RC2, and the capacitance C2 are calculated from:XC2=Zright·sin(phi) RC2=Zright·cos(phi)−R3       C    2    =      1          2      ·      π      ·      f      ·              X        C2            
The Quality Factor (Q) is calculated from Q2=XC2/RC2.
The two cases described in detail above illustrate that the process of measuring the capacitance and Q of a capacitor conventionally involves measuring the displacement current and the phase shift of the displacement current through the capacitor when a known alternating current signal is applied.
The features common to the Wien bridge and the Schering bridge are that an alternating current signal generator is connected to the series combination of the unknown capacitor and a known impedance. By balancing the bridge, the displacement current and phase shift of the displacement current in the unknown capacitor are determined from which the parameters of the unknown capacitor are calculated.
Vehicles in volume production include a fluid filled bladder placed under a seat cushion to sense the weight of a seat occupant by measuring the pressure in the fluid. Other sensors and mechanisms for determining seat occupant weight are in volume production.
A general object of this invention is to provide sensors for seat occupant sensing and associated calculation and decision making means for automotive vehicles, which also overcomes certain disadvantages of the prior art.